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Neptune has an equatorial radius of 24,764 ± 15 km and a polar radius of 24,341 ± 30 km.[1]

An orbital pole is either end of an imaginary line running through the center of an orbit perpendicular to the orbital plane, projected onto the celestial sphere. It is similar in concept to a celestial pole but based on the planet's orbit instead of the planet's rotation.

The north orbital pole of a celestial body is defined by the right-hand rule: If you curve the fingers of your right hand along the direction of orbital motion, with your thumb extended parallel to the orbital axis, the direction your thumb points is defined to be north.

At right is a snapshot of the planetary orbital poles.[2] The field of view is about 30°. The yellow dot in the centre is the Sun's North pole. Off to the side, the orange dot is Jupiter's orbital pole. Clustered around it are the other planets: Mercury in pale blue (closer to the Sun than to Jupiter), Venus in green, the Earth in blue, Mars in red, Saturn in violet, Uranus in grey partly underneath Earth and Neptune in lavender. Dwarf planet Pluto is the dotless cross off in Cepheus.

"The location of a newly discovered moon, designated S/2004 N 1, orbiting Neptune, is seen in this composite Hubble Space Telescope handout image [at right] taken in August 2009. The new moon is the 14th known moon to be circling the distant blue-green planet."[3]

"Estimated to be about 12 miles (20 km) in diameter, the moon is located about 65,400 miles (105,251 km) from Neptune [left image]."[3]

"Images taken by NASA's Voyager 2 spacecraft unveiled the second largest moon, Proteus, and five smaller moons, Naiad, Thalassa, Despina, Galatea and Larissa."[3]

"Ground-based telescopes found Halimede, Laomedeia, Sao and Nestor in 2002. Sister moon Psamathe turned up a year later."[3]

"The newly found moon, designated S/2004 N 1, is located between Larissa and Proteus. It orbits Neptune in 23 hours."[3]

"The moon is so small (no more than 12 miles across) and dim, it was missed by NASA's Voyager 2 spacecraft cameras when the probe flew by Neptune in 1989. Several other moons that were discovered by Voyager appear in this 2009 image, along with a circumplanetary structure known as ring arcs."[4]

"Mark Showalter of the SETI Institute discovered S/2004 N 1 in July 2013. He analyzed over 150 archival Neptune photographs taken by Hubble from 2004 to 2009. The same white dot appeared over and over again. He then plotted a circular orbit for the moon, which completes one revolution around Neptune every 23 hours."[4]

At the time of the 1989 Voyager 2 flyby, the planet's southern hemisphere possessed a Great Dark Spot. In 1989, the Great Dark Spot, an anti-cyclonic storm system spanned 13000×6600 km,[6] was discovered by NASA's Voyager 2 spacecraft. Some five years later, on 2 November 1994, the Hubble Space Telescope did not see the Great Dark Spot on the planet. Instead, a new storm similar to the Great Dark Spot was found in the planet's northern hemisphere.[7]

The Scooter is another storm, a white cloud group farther south than the Great Dark Spot. Its nickname is due to the fact that when first detected in the months before the 1989 Voyager 2 encounter it moved faster than the Great Dark Spot.[8] Subsequent images revealed even faster clouds.

The Small Dark Spot is a southern cyclonic storm, the second-most-intense storm observed during the 1989 encounter. It initially was completely dark, but as Voyager 2 approached the planet, a bright core developed and can be seen in most of the highest-resolution images.[9]

The persistence of companion clouds shows that some former dark spots may continue to exist as cyclones even though they are no longer visible as a dark feature. Dark spots may dissipate when they migrate too close to the equator or possibly through some other unknown mechanism.[10]

The upper-level clouds occur at pressures below one bar, where the temperature is suitable for methane to condense.

High-altitude clouds on Neptune have been observed casting shadows on the opaque cloud deck below. There are also high-altitude cloud bands that wrap around the planet at constant latitude. These circumferential bands have widths of 50–150 km and lie about 50–110 km above the cloud deck.[11]

Because of seasonal changes, the cloud bands in the southern hemisphere of Neptune have been observed to increase in size and albedo. This trend was first seen in 1980 and is expected to last until about 2020. The long orbital period of Neptune results in seasons lasting forty years.[12]

Neptune has the strongest sustained winds of any planet in the Solar System, with recorded wind speeds as high as 2,100 kilometres per hour (1,300 mph).[13]

On Neptune winds reach speeds of almost 600 m/s—nearly attaining supersonic flow.[13] More typically, by tracking the motion of persistent clouds, wind speeds have been shown to vary from 20 m/s in the easterly direction to 325 m/s westward.[14] At the cloud tops, the prevailing winds range in speed from 400 m/s along the equator to 250 m/s at the poles.[15] Most of the winds on Neptune move in a direction opposite the planet's rotation.[8] The general pattern of winds showed prograde rotation at high latitudes vs. retrograde rotation at lower latitudes. The difference in flow direction is believed to be a "skin effect" and not due to any deeper atmospheric processes.[16] At 70° S latitude, a high-speed jet travels at a speed of 300 m/s.[16]

The fourth image down on the right shows a dark vortex on Neptune.

"The full visible-light image at left shows that the dark feature resides near and below a patch of bright clouds in the planet's southern hemisphere. The dark spot measures roughly 3,000 miles (4,800 kilometers) across. Other high-altitude clouds can be seen at the planet's equatorial region and polar regions."[17]

"The full-color image at top right is a close-up of the complex feature. Pancake-shaped clouds above the spot form when ambient air is perturbed and diverted upward over the vortex. The vortex is a high-pressure system."[17]

"The image at bottom right shows that the vortex is best seen at blue wavelengths. Only Hubble has the high resolution required for identifying such weather features on distant Neptune."[17]

"Though similar features were seen during the Voyager 2 flyby of Neptune in 1989 and by Hubble in 1994, this vortex is the first one observed on the planet in the 21st century."[17]

"In September 2015, the Outer Planet Atmospheres Legacy (OPAL) program, a long-term Hubble Space Telescope project that annually captures global maps of the outer planets, revealed a dark spot close to the location of the bright clouds, which had been tracked from the ground. By viewing the vortex a second time, the new Hubble images, taken by Wide Field Camera 3 on May 16, 2016, confirm that OPAL really detected a long-lived feature."[17]

The second image down on the left show Neptune as seen from Earth about 8 February 2015.

At right is a set of images from different years for Neptune. These images "show that Neptune's brightness has increased significantly since 1996. The rise is due to an increase in the amount of clouds observed in the planet's southern hemisphere. These increases may be due to seasonal changes caused by a variation in solar heating. Because Neptune's rotation axis is inclined 29 degrees to its orbital plane, it is subject to seasonal solar heating during its 164.8-year orbit of the Sun. This seasonal variation is 900 times smaller than experienced by Earth because Neptune is much farther from the Sun. The rate of seasonal change also is much slower because Neptune takes 165 years to orbit the Sun. So, springtime in the southern hemisphere will last for several decades! Remarkably, this is evidence that Neptune is responding to the weak radiation from the Sun. These images were taken in visible and near-infrared light by Hubble's Wide Field and Planetary Camera 2."[19]

This picture of Neptune was produced from the last whole planet images taken through the green and orange filters on the Voyager 2 narrow angle camera. Credit: NASA Jet Propulsion Laboratory.

"This picture of Neptune was produced from the last whole planet images taken through the green and orange filters on the Voyager 2 narrow angle camera. The images were taken at a range of 4.4 million miles from the planet, 4 days and 20 hours before closest approach."

"The picture shows the Great Dark Spot and its companion bright smudge; on the west limb the fast moving bright feature called Scooter and the little dark spot are visible. These clouds were seen to persist for as long as Voyager's cameras could resolve them. North of these, a bright cloud band similar to the south polar streak may be seen."[20]

This false color photograph of Neptune was made from Voyager 2 images taken through three filters: blue, green, and a filter that passes light at a wavelength that is absorbed by methane gas. Credit: Voyager Mission, NASA/JPL.

"Neptune is shown in natural color with its largest satellite, Triton, as it orbits the planet. Triton orbits in a "backwards" or retrograde path relative to the other major satellites, and is opposite to Neptune's rotation. Zooming into Neptune, the colors were enhanced to show the subtle details in Neptune's atmosphere. The spectral region of light is changed from the visible to special methane bands in the near-infrared resulting in Neptune's atmosphere becoming very dark, except for high-altitude clouds. Coming into view are Neptune's four smaller moons: Proteus, Larissa, Galatea and Despina."[21]

"This false color photograph [on the left] of Neptune was made from Voyager 2 images taken through three filters: blue, green, and a filter that passes light at a wavelength that is absorbed by methane gas. Thus, regions that appear white or bright red are those that reflect sunlight before it passes through a large quantity of methane. The image reveals the presence of a ubiquitous haze that covers Neptune in a semitransparent layer. Near the center of the disk, sunlight passes through the haze and deeper into the atmosphere, where some wavelengths are absorbed by methane gas, causing the center of the image to appear less red. Near the edge of the planet, the haze scatters sunlight at higher altitude, above most of the methane, causing the bright red edge around the planet. By measuring haze brightness at several wavelengths, scientists are able to estimate the thickness of the haze and its ability to scatter sunlight. The image is among the last full disk photos that Voyager 2 took before beginning its endless journey into interstellar space."[22]

This is a Voyager 2 image of the back side of Neptune. Credit: Voyager 2, NASA.

A trace amount of methane is also present. Prominent absorption bands of methane occur at wavelengths above 600 nm, in the red and infrared portion of the spectrum. As with Uranus, this absorption of red light by the atmospheric methane is part of what gives Neptune its blue hue,[23] although Neptune's vivid azure differs from Uranus's milder cyan. Since Neptune's atmospheric methane content is similar to that of Uranus, some unknown atmospheric constituent is thought to contribute to Neptune's colour.[24]

On the lower right is a Voyager 2 image of the other side of Neptune from the Great Dark Spot.

The snapshots of Neptune were taken at roughly 4-hour intervals, offering a full view of the blue-green planet. Credit: NASA/ESA/Hubble Heritage Team (STScI/AURA).

On July 12, 2011, Neptune "has arrived at the same location in space where it was discovered nearly 165 years ago. To commemorate the event, NASA's Hubble Space Telescope has taken these "anniversary pictures" of the blue-green giant planet."[25]

"Neptune is the most distant major planet in our solar system. German astronomer Johann Galle discovered the planet on September 23, 1846. At the time, the discovery doubled the size of the known solar system. The planet is 2.8 billion miles (4.5 billion kilometers) from the Sun, 30 times farther than Earth. Under the Sun's weak pull at that distance, Neptune plods along in its huge orbit, slowly completing one revolution approximately every 165 years."[25]

"These four Hubble images of Neptune were taken with the Wide Field Camera 3 on June 25-26, during the planet's 16-hour rotation. The snapshots were taken at roughly four-hour intervals, offering a full view of the planet. The images reveal high-altitude clouds in the northern and southern hemispheres. The clouds are composed of methane ice crystals."[25]

"The giant planet experiences seasons just as Earth does, because it is tilted 29 degrees, similar to Earth's 23-degree-tilt. Instead of lasting a few months, each of Neptune's seasons continues for about 40 years."[25]

"The snapshots show that Neptune has more clouds than a few years ago, when most of the clouds were in the southern hemisphere. These Hubble views reveal that the cloud activity is shifting to the northern hemisphere. It is early summer in the southern hemisphere and winter in the northern hemisphere."[25]

"In the Hubble images, absorption of red light by methane in Neptune's atmosphere gives the planet its distinctive aqua color. The clouds are tinted pink because they are reflecting near-infrared light."[25]

"A faint, dark band near the bottom of the southern hemisphere is probably caused by a decrease in the hazes in the atmosphere that scatter blue light. The band was imaged by NASA's Voyager 2 spacecraft in 1989, and may be tied to circumpolar circulation created by high-velocity winds in that region."[25]

At right are three images of Neptune using infrared astronomy. "Thermal images of planet Neptune taken with VISIR on ESO's Very Large Telescope, obtained on 1 and 2 September 2006. These thermal images show a 'hot' south pole on Neptune. These warmer temperatures provide an avenue for methane to escape out of the deep atmosphere. Scientists say Neptune's south pole is 'hotter' than anywhere else on the planet by about 10°C. The average temperature on Neptune is about minus 200 degrees Celsius. The upper left image samples temperatures near the top of Neptune's troposphere (near 100 mbar pressure). The hottest temperatures are located at the lower part of the image at Neptune's south pole (see the graphic at the upper right). The lower two images, taken 6.3 hours apart, sample temperatures at higher altitudes in Neptune's stratosphere. They do show generally warmer temperatures near, but not at, the south pole. In addition they show a warm area which can be seen in the lower left image and rotated completely around the planet in the lower right image."[26]

"Neptune has never looked so clear in infrared light. Surprisingly, Neptune radiates about twice as much energy as it receives from the sun. A fascinating feature of the above photograph is that it was taken far from distant Neptune, through the Earth's normally blurry atmosphere. The great clarity of this recently released image was made possible by "rubber mirror" adaptive optics technology. Here, mirrors in the Palomar High Angular Resolution Observer (PHARO) instrument connected to the 200-inch Hale Telescope flex to remove the effects of turbulence in the Earth's atmosphere."[27]

The second image down on the right shows Neptune at 1.5 microns exhibiting two distinct bands.

Neptune's atmosphere is composed primarily of hydrogen and helium, along with traces of hydrocarbons and possibly nitrogen, contains a higher proportion of "ices" such as water, ammonia, and methane. Traces of methane in the outermost regions in part account for the planet's blue appearance.[29]

At high altitudes, Neptune's atmosphere is 80% hydrogen and 19% helium.[30] A trace amount of methane is also present. Prominent absorption bands of methane occur at wavelengths above 600 nm, in the red and infrared portion of the spectrum. As with Uranus, this absorption of red light by the atmospheric methane is part of what gives Neptune its blue hue,[23] although Neptune's vivid azure differs from Uranus's milder cyan. Since Neptune's atmospheric methane content is similar to that of Uranus, some unknown atmospheric constituent is thought to contribute to Neptune's colour.[29]

Neptune's atmosphere is sub-divided into two main regions; the lower troposphere, where temperature decreases with altitude, and the stratosphere, where temperature increases with altitude. The boundary between the two, the tropopause, occurs at a pressure of 0.1 bars (10 kPa).[31] The stratosphere then gives way to the thermosphere at a pressure lower than 10−5 to 10−4 microbars (1 to 10 Pa).[31] The thermosphere gradually transitions to the exosphere.

Neptune's thermosphere is at an anomalously high temperature of about 750 K.[32][33]

"In 2007 it was discovered that the upper troposphere of Neptune's south pole was about 10 °C warmer than the rest of Neptune, which averages approximately -200 °C (70 K).[34] The warmth differential is enough to let methane, which elsewhere lies frozen in Neptune's upper atmosphere, leak out as gas through the south pole and into space. The relative "hot spot" is due to Neptune's axial tilt, which has exposed the south pole to the Sun for the last quarter of Neptune's year, or roughly 40 Earth years. As Neptune slowly moves towards the opposite side of the Sun, the south pole will be darkened and the north pole illuminated, causing the methane release to shift to the north pole.[35]

The atmosphere of Neptune, similar to Uranus, consists of mainly hydrogen, methane, and helium. Below it is a liquid hydrogen layer including helium and methane. The lower layer is liquid hydrogen compounds, oxygen, and nitrogen. It is believed that the planet core comprises rock and ice. Average density, as well as the greatest proportion of core per planet size, is the greatest among the gaseous planets."[36]

"The discovery of [Neptune]'s non-dipolar, non-axisymmetric magnetic [field contributes to destroying] the picture-established by Earth, Jupiter and Saturn-that planetary magnetic fields are dominated by axial dipoles."[37]

The "convective-region geometry produces magnetic fields similar in morphology to [that of] Neptune. [The field is] non-dipolar and non-axisymmetric, and [results] from a combination of the stable fluid's response to electromagnetic stress and the small length scales imposed by the thin shell."[37]

The "rotation axis of [Neptune] is inclined by only 29° to the orbital plane [...] The magnetic dipole axis of Neptune is tilted at an angle of 47° to the spin axis of the planet. The extrapolated near-equatorial surface field is 1.42 µT, corresponding to a magnetic moment (equatorial surface field times radius cubed) of 2.16 x 1017 Tm3 close to 27 times greater than the terrestrial magnetic moment. The quadrupole moment if Neptune is quite large and makes a greater contribution to the surface magnetic field than at any other planet. The most forward portion of the magnetopause is estimated to lie on average at about 26 Neptunian radii in front of the planet, and of the bow shock at about 34 Neptune radii."[38]

Neptune is never visible to the naked eye, having a brightness between magnitudes +7.7 and +8.0,[39][40]

Neptune was the Roman god of water and the sea[41] in Roman mythology and religion. He is the counterpart of the Greek god Poseidon. In the Greek-influenced tradition, Neptune was the brother of Jupiter [Italic Neptune has been securely identified as a god of freshwater sources as well as the sea.][41]

Syncretic traces of a Lybian/Punic agrarian god of fresh water sources, with the epithet Frugifer, "fruit-bearer"; have been enumerated”[42]

The German scholar H. Petersmann proposed an etymology from IE rootstem *nebh- related to clouds and foggs. The concept would be close to that expressed in the name of Greek god Uranus.

Indo-European people, having no direct knowledge of the sea as they originated from inland areas, reused the theology of a deity originally either chthonic or wielding power over inland freshwaters as the god of the sea.[43] This feature has been preserved particularly well in the case of Neptune who was definitely a god of springs, lakes and rivers before becoming also a god of the sea, as is testified by the numerous findings of inscriptions mentioning him in the proximity of such locations. Servius the grammarian also explicitly states Neptune is in charge of all the rivers, springs and waters.[44]

"I find it most useful to refer to the eight planets Mercury through Neptune as the "classical planets"."[45] "By restricting the new definition to the eight existing “classical planets,” the second resolution implied that dwarf planets were a subcategory of planets, too."[46]